Touch or proximity sensing system and method

ABSTRACT

A system for sensing touch or proximity include: a first number of input terminals configured to couple one or more capacitive sensors, a second number of transferring units configured to transfer charges from the one or more capacitive sensors through the first number of input terminals in transferring phases of cycles of the one or more capacitive sensor, wherein at least one of the first and second numbers is equal to or greater than two, and a first switching unit, coupled between the first number of input terminals and the second number of transferring units, configured to selectively electrically couple any one of the first number of input terminals to any one of the second number of transferring units in the transferring phases.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S Pat. Application No. 17/687,768, filed Mar. 7, 2022, which is a continuation of U.S. Pat. Application No. 16/741,614, filed Jan. 13, 2020, now U.S. Pat. No. 11,271,565, which is a continuation of PCT Application CN2019/100501, filed Aug. 14, 2019, each of which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to a sensing system and, more particularly, to a touch or proximity sensing system based on capacitance change of capacitive sensors.

BACKGROUND

Capacitive sensors are widely used in various applications, for example, in human interface applications to sense touch or proximity of a body, and in motor applications to determine a position of a rotatable shaft by determining proximity of a conductive object to a capacitive sensor mounted on the shaft. Touch or proximity to the capacitive sensor is determined by detecting a capacitance change between two opposite electrodes thereof.

SUMMARY

The present disclosure relates to touch or proximity sensing systems with improved noise immunity, configuration flexibility and averaged measurement errors. The touch or proximity is determined by detecting a capacitance change between two opposite electrodes of a capacitive sensor. The capacitance between the two electrodes of the capacitive sensor is measured by charging the capacitive sensor to a predetermined voltage and measuring the amount of charges charged to the capacitive sensor. The measurement includes transferring the charges to a sample capacitor by discharging the capacitive sensor.

In one example, a system for sensing touch or proximity is described. The system includes a first number of input terminals configured to couple one or more capacitive sensors, a second number of transferring units configured to transfer charges from the one or more capacitive sensors through the first number of input terminals in transferring phases of cycles of the one or more capacitive sensor, wherein at least one of the first and second numbers is equal to or greater than two, and a first switching unit, coupled between the first number of input terminals and the second number of transferring units, configured to selectively electrically couple any one of the first number of input terminals to any one of the second number of transferring units in the transferring phases.

In another example, the present disclosure provides a method of operating a touch or proximity sensing system including a first number of input terminals and a second number of transferring units. The method includes repeatedly charging one or more capacitive sensors in charging phases of cycles of the one or more capacitive sensors, and selectively configuring any one of a second number of transferring units to transfer charges from any one of the first number of capacitive sensor in transferring phases of the cycles, wherein at least one of the first and second numbers is equal to or greater than two.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a sensing system in accordance with a first embodiment of the present disclosure;

FIG. 2 is a timing diagram illustrating changes of voltage differences across first and second capacitive sensors during operation of the sensing system of FIG. 1 in accordance with an example;

FIGS. 3 a and 3 b are respective schematic circuit and block diagrams modeling how an input noise affects the sensing system of FIG. 1 ;

FIG. 4 illustrates sampling the input noise with the sample clock of FIGS. 3 a and 3 b in time and frequency domains;

FIG. 5 illustrates the sample clock with different phase shifts with respect to the input noise in time domain;

FIG. 6 is a simulation diagram illustrating input current of the transferring unit affected by the input noise with different phase shifts with respect to the sample clock;

FIG. 7 is a timing diagram illustrating changes of voltage differences across the first and second capacitive sensors during operation of the sensing system of FIG. 1 in accordance with another example;

FIG. 8 is a schematic block diagram of a sensing system in accordance with a second embodiment of the present disclosure;

FIG. 9 is a schematic block diagram of a sensing system in accordance with a third embodiment of the present disclosure;

FIG. 10 is a timing diagram illustrating changes of voltage differences across first and second capacitive sensors during operation of the sensing system of FIG. 9 in accordance with an example;

FIG. 11 is a schematic block diagram of a sensing system in accordance with a fourth embodiment of the present disclosure;

FIG. 12 is a timing diagram illustrating changes of voltages at the first and second input terminals of the sensing system of FIG. 11 during operation of the sensing system in accordance with an example;

FIG. 13 is a timing diagram illustrating changes of voltages at the first and second input terminals of the sensing system of FIG. 11 during the operation of the sensing system in accordance with another example;

FIG. 14 is a flow chart of a method for sensing touch or proximity in accordance with a fifth embodiment of the present disclosure;

FIG. 15 is a flow chart of a method for sensing touch or proximity in accordance with a sixth embodiment of the present disclosure; and

FIG. 16 is a flow chart of a method for sensing touch or proximity in accordance with a seventh embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates to a touch or proximity sensing system that determines touch or proximity by detecting a capacitance change between two opposite electrodes of at least one capacitive sensor. The capacitance between the two electrodes of the capacitive sensor is measured by charging and discharging the capacitive sensor, the discharging including transferring charges charged to the capacitive sensor to a sample capacitor. The capacitance is determined based on cycles to charge a voltage difference across the sample capacitor to a predetermined voltage.

Referring now to FIG. 1 , a schematic block diagram of a sensing system 100 for sensing touch or proximity with at least a first capacitive sensor 102 a in accordance with a first embodiment of the present disclosure is shown. The first capacitive sensor 102 a includes a first end coupled to the sensing system 100 and a second end opposite to the first end coupled to ground. The sensing system 100 is configured to charge a number of charges to the first capacitive sensor 102 a through the first end thereof in charging phases of successive cycles, and transfer the charges to a sample capacitor in transferring phases of the successive cycles by discharging the first capacitive sensor 102 a, to sense a capacitance change of the first capacitive sensor 102 a.

The sensing system 100 includes a first number of input terminals 104 configured to couple to one or more capacitive sensors, such as the first capacitive sensor 102 a, and a second number of transferring units 106 configured to transfer the charges from the one or more capacitive sensors through the first number of input terminals 104 in the transferring phases, wherein at least one of the first and second numbers is equal to or greater than two. The sensing system 100 further includes a first switching unit 108 a, coupled between the first number of input terminals 104 and the second number of transferring units 106, configured to selectively electrically couple any one of the first number of input terminals 104 to any one of the second number of transferring units 106.

In a preferred embodiment, the first number of input terminals 104 include a first input terminal 104 a configured to couple the first capacitive sensor 102 a, and the second number of transferring units 106 include first and second transferring units 106 a and 106 b. In a preferred embodiment, each of the first and second transferring units 106 a and 106 b is a current mirror. In a preferred embodiment, the current mirror is a class AB current mirror. The sensing system 100 further includes a second switching unit 108 b, coupled to the second number of transferring units 106. In one example, the first and second switching units 108 a and 108 b are configured to alternately electrically couple one of the first and second transferring units 106 a and 106 b between the first input terminal 104 a and a first sample capacitor 110 a in transferring phases of the first capacitive sensor 102 a, to transfer charges charged to the first capacitive sensor 102 a to the first sample capacitor 110 a. The first sample capacitor 110 a can either be a part of the sensing system 100 or separate from the sensing system 100. The touch or proximity to the first capacitive sensor 102 a is determined based on cycles for charging a voltage difference across the first sample capacitor 110 a to a predetermined voltage. In a preferred implementation, the first number of input terminals 104 further includes a second input terminal 104 b. In a preferred implementation, the second input terminal 104 b is configured to receive charges from a second capacitive sensor 102 b. The touch or proximity to the second capacitive sensor 102 b is determined in a same or similar manner.

In one example, the sensing system 100 includes first and second charging switches 112 a and 112 b respectively coupled between a voltage source V_(reg) 114 and the first and second input terminals 104 a and 104 b respectively configured to charge the first and second capacitive sensors 102 a and 102 b during charging phases thereof. The first switching unit 108 a includes first to fourth transferring switches 116 a to 116 d coupled between the first number of input terminals 104 and the second number of transferring units 106 for selectively electrically coupling any one of the first and second input terminals 104 a and 104 b to any one of the first and second transferring units 106 a and 106 b. The second switching unit 108 b includes fifth to eighth transferring switches 116 e to 116 h coupled between the second number of transferring units 106 and the first and a second sample capacitors 110 a and 110 b for selectively electrically coupling any one of the second number of transferring units 106 to any one of the first and second sample capacitors 110 a and 110 b. The first and second charging switches 112 a and 112 b can be transistors operating between ON and OFF status respectively controlled by corresponding first and second charging signals S_(1_A) and S_(1_B), the first to fourth transferring switches 116 a to 116 d can be transistors operating between ON and OFF status respectively controlled by corresponding first to fourth transferring signals S_(2_A), S_(2_B), S_(3_A) and S_(3_B), and the fifth to eighth transferring switches 116 e to 116 h can be transistors operating between ON and OFF status respectively controlled by the first to fourth transferring signals S_(2_A), S_(2_B), S_(3_A) and S_(3_B). In one implementation, the charging and transferring signals are generated by a signal generator (not shown) based on a clock signal. The relationship between the clock signal and the charging or transferring signals can be configurable by a user through the signal generator.

Taking the first capacitive sensor 102 a for example, during operation, the first charging signal S_(1_A) is asserted in charging phases of successive cycles, and the first and second transferring signals S_(2_A) and S_(3_A) alternately asserted in transferring phases of the successive cycles to alternately configure one of the first and second transferring units 106 a and 106 b to transfer charges from the first capacitive sensor 102 a to the first sample capacitor 110 a, which averages errors caused by the first and second transferring units 106 a and 106 b due to, for example, random telegraph signal (RTS) noise, current mirror nonlinearity, etc. Therefore, accuracy of capacitance change sensing is improved.

FIG. 2 is a timing diagram 200 illustrating changes of voltages V_(X_A) and V_(X_B) at the first and second input terminals 104 a and 104 b during operation of the sensing system 100 of FIG. 1 in accordance with an example. The timing diagram 200 demonstrates the voltages V_(X_A) and V_(X_B) respectively at 202 and 204, the first and second charging signals S_(1_A) and S_(1_B) respectively at 206 and 208, and the first to fourth transferring signals S_(2_A), S_(3_A), S_(2_B) and S_(3_B) respectively at 210 to 216. The first and second charging signals S_(1_A) and S_(1_B) configure the first and second capacitive sensors 102 a and 102 b to be charged respectively in first and second half of each cycle. In an example, the first and second transferring signals S_(2_A) and S_(3_A) configure the first and second transferring units 106 a and 106 b to alternately transfer the charges received at the first input terminal 104 a to the first sample capacitor 110 a in the second half of each cycle. The third and fourth transferring signals S_(2_B) and S_(3_B) configure the first and second transferring units 106 a and 106 b to alternately transfer the charges received at the second input terminal 104 b to the second sample capacitor 110 b in the first half of each cycle. In a preferred embodiment, a gap time 218 is provided between charging and transferring to ensure break-before-make switch action.

Referring back to FIG. 1 , the sensing system 100 further includes a determining unit 118 coupled to at least the first sample capacitor 110 a. In a preferred embodiment, the sensing system 100 performs first and second conversions, in each conversion a voltage difference across the first sample capacitor 110 a is charged to a first trip voltage V_(trip1), wherein the charging and transferring phases in the second conversion are half-cycle phase shifted with respect to the corresponding charging and transferring phases in the first conversion, which improves noise immunity of the sensing system 100 against noise injection. The determining unit 118 generates a first signal N_(1x) indicative of a number of the cycles in the first conversion and a second signal N_(2x) indicative of a number of the cycles in the second conversion, wherein the touch or proximity to the first capacitive sensor 102 a is determined based on a final count N_(x) which is a combination of the first and second signals N_(1x) and N_(2x). In one example, the final count N_(x) is defined in accordance with the equation below:

$\begin{matrix} {{N_{x} = 2 \cdot N_{1x} \cdot N_{2x}}/\left( {N_{1x} + N_{2x}} \right)} & \text{­­­(1)} \end{matrix}$

The touch or proximity to the second capacitive sensor 102 b is determined in a same manner based on two conversions with half-cycle phase shift. In one implementation, the sensing system 100 performs sequential scanning over the first and second capacitive sensors 102 a and 102 b to determine touch or proximity respectively to the first ends 104 a and 104 b of the first and second capacitive sensors 102 a and 102 b in a situation that the first and second capacitive sensors 102 a and 102 b respectively correspond to first and second buttons.

Referring to FIG. 2 , the first charging signal that controls the first charging switch 112 a in the second conversion, marked as S_(1_A′), is demonstrated at 220, which shows half-cycle phase shift with respect to the first charging signal S_(1_A) 206 in the first conversion. The rest signals are also phase shifted by half cycle in a same manner in the second conversion.

In a preferred embodiment, the determining unit 118 includes a first comparator 120 a that compares the voltage difference across the first sample capacitor 110 a with the first trip voltage V_(trip1), and a first counter (not shown) coupled to the first comparator 120 a. The first counter counts the cycles in each of the first and second conversions and respectively generates the first and second signals N_(1x) and N_(2x) when the voltage difference across the first sample capacitor 110 a reaches the first trip voltage V_(trip1) in each of the first and second conversions. In a preferred embodiment, the first sample capacitor 110 a is coupled between an input terminal of the first comparator 120 a and ground, therefore a voltage difference across the first sample capacitor 120 a is determined by determining a voltage at the input terminal of the first comparator 120 a. Similarly, the determining unit 118 further includes a second comparator 120 b that compares the voltage difference across the second sample capacitor 110 b with V_(trip2), and a second counter (not shown) coupled to the second comparator 120 b. In a preferred embodiment, the first and second trip voltages V_(trip1) and V_(trip2) are equal to a same trip voltage V_(trip).

The noise injection from electrode to system can be modeled as those in FIGS. 3 a and 3 b , which respectively are simplified equivalent schematic circuit and block diagrams 300 and 302 modeling how an input noise 304 injected through the first input terminal 104 a affects the sensing system 100 of FIG. 1 . The noise injection can be extracted as a natural sampling process plus an integrating process. The input noise 304 represented by a sine signal can be regarded as an additional current source Inoise, and is injected to the sensing system 100 with the touch or proximity to the first sensor capacitor 102 a in the transferring phases. The input noise is sampled by a sample clock 306, e.g. the first transferring signal S2_A, and conveyed to and integrated by an integrator 308, e.g. the first sample capacitor 110 a. Qs is the noise charge injects to system. The integrator 308 can be regarded as a low pass filter, thus only low frequency components of the input noise matter.

FIG. 4 illustrates sampling the input noise 304 with the sample clock 306 of FIGS. 3 a and 3 b in time and frequency domains. In the time domain, the input noise Inoise 304 is represented by a quasi-sine signal x (t) with a frequency f_(noise) in time domain and by X(f) in frequency domain, and the sample clock 306 is represented by a square wave signal s(t) with a frequency f_(chg) in time domain and by S(f) in frequency domain. The sine waveform of the input noise Inoise 304 with certain distortions due to the resistance of the first transferring unit 106 a and the controller 108 are not linear, and a maximum current of the input noise Inoise 304 is limited. A sampled Inoise 400 is represented by xs(t) in time domain and by XS(f) in frequency domain. As the example shown in FIG. 4 , the sampled Inoise is formed by multiplication of the input noise Inoise and sample clock in time domain and convolution of the input noise Inoise and sample clock in frequency domain.

In an example, in frequency domain, the frequency f_(noise) of the input noise Inoise may range from 150 kHz to 80 MHz, and the frequency f_(chg) of the sample clock usually ranges from 4 kHz to 4 MHz, and in some situations, both are greater than 100 kHz. Therefore, only f_(noise)-n. f_(chg) frequency components of the input noise Inoise matter, where n is a natural number. In addition, if the duty cycle of the sample clock is about 50%, impact of the frequency components of the input noise Inoise with frequencies equal to even harmonics of the frequency f_(chg) of the sample clock is null.

In time domain, the input noise Inoise represented by x(t) is defined in accordance with the equation below:

$\begin{matrix} {x(t) = Asin\left( {\omega t} \right)} & \text{­­­(2)} \end{matrix}$

wherein ω=2πf_(noise).

s(t) can be represented by Fourier Series as the equation below:

$\begin{matrix} {s(t) = C_{0} + {\sum_{n = 1}^{\infty}{2C_{n}\mspace{6mu} cos\left( {n\omega_{s}t} \right)}}} & \text{­­­(3)} \end{matrix}$

wherein

$C_{0} = \frac{\tau}{T_{s}}\mspace{6mu}\text{and}C_{n} = f_{s}\tau sinc\left( {\pi nf_{s}\tau} \right),\mspace{6mu}\omega_{s} = 2\pi f_{chg}.$

If the duty cycle of the sample clock is 50%, τ=0.5 Ts. Therefore, C₀=½ and Cn=0.5sinc(nπ/2). Hence, the sampled Inoise xs (t) is:

$\begin{matrix} {xs(t) = x(t) \cdot s(t) = Asin\left( {\omega t} \right)\left\lbrack {\frac{1}{2} + {\sum_{n = 1}^{\infty}{sinc\left( \frac{n\pi}{2} \right) \cdot cos\left( {n\omega_{s}t} \right)}}} \right\rbrack} & \text{­­­(4)} \end{matrix}$

In Equation (4), frequency components of interest are:

$\begin{array}{l} {\omega - \omega_{s}:\frac{1}{2}Asinc\left( \frac{\pi}{2} \right)\sin\left( {\omega - \omega_{s}} \right)t = \frac{A}{\pi}\sin\left( {\omega - \omega_{s}} \right)t\omega - 3\omega_{s}:} \\ {\frac{1}{2}Asinc\left( \frac{3\pi}{2} \right)\sin\left( {\omega -} \right)} \\ {\left( {3\omega_{s}} \right)t = - \frac{A}{3\pi}\sin\left( {\omega - 3\omega_{s}} \right)t\quad,\mspace{6mu}\text{and}} \end{array}$

$\begin{array}{l} {\omega - \omega_{s}:\frac{1}{2}Asinc\left( \frac{\pi}{2} \right)\sin\left( {\omega - \omega_{s}} \right)t = \frac{A}{\pi}\sin\left( {\omega - \omega_{s}} \right)t\omega - 3\omega_{s}:} \\ {\frac{1}{2}Asinc\left( \frac{3\pi}{2} \right)\sin\left( {\omega -} \right)} \\ {\left( {3\omega_{s}} \right)t = - \frac{A}{3\pi}\sin\left( {\omega - 3\omega_{s}} \right)t\quad.} \end{array}$

If delay s(t) by 180° in phase,

$\begin{matrix} {s\left( {t + \frac{T_{S}}{2}} \right) = C_{0} + {\sum_{n = 1}^{\infty}{2C_{n}\mspace{6mu} cos\left( {n\omega_{s}t + \pi \cdot n} \right)}}} & \text{­­­(5)} \end{matrix}$

Hence, the delayed sampled Inoise is:

$\begin{matrix} {xs^{\prime}(t) = x(t) \cdot s\left( {t + \frac{T_{s}}{2}} \right)} & \text{­­­(6)} \end{matrix}$

In Equation (6), frequency components of interest are:

$\omega - \omega_{s}: - \frac{1}{2}Asinc\left( \frac{\pi}{2} \right)\mspace{6mu}\sin\left( {\omega - \omega_{s}} \right)t = - \frac{A}{\pi}\sin\left( {\omega - \omega_{s}} \right)\mspace{6mu} t,\mspace{6mu}\text{and}$

$\omega - 3\omega_{s}: - \frac{1}{2}Asinc\left( \frac{3\pi}{2} \right)\sin\left( {\omega - 3\omega_{s}} \right)t = \frac{A}{3\pi}\sin\left( {\omega - 3\omega_{s}} \right)t.$

From the deduction above, it is proved that s(t) and s(t+T_(s)/2) have opposite magnitude for those frequency components of interests, thus the impact of the input noise in the two situations can be cancelled by each other.

FIG. 5 is a waveform diagram showing the sample clock 306 and the input noise 304 of FIG. 3 under different phase shifts with each other. In one example, the frequency f_(chg) of the sample clock 306 and frequency f_(noise) of the input noise Inoise 304 are equal, and little or no distortion is caused by the resistance of the first transferring unit 106 a and the controller 108. The input noises Inoise with different phase shifts with reference to the sample clock 306 are demonstrated at 500 to 504.

In (a) where the sample clock 306 and input noise Inoise 500 are in the same phase, the sampled Inoise 506 is zero. In (b) where the phase shift between the sample clock 306 and input noise Inoise 502 is 90_(◦), the sampled Inoise 508 is positive. In (c) where the phase shift between the sample clock 306 and input noise Inoise 504 is 270◦, the sampled Inoise is negative. The sampled Inoise 508 and 510 in (b) and (c) are same in magnitude but with opposite direction.

FIG. 6 is a simulation diagram illustrating how the input noise with different phase shift affects an input charge of the first transferring unit 106 a. The input charge with no affect from the input noise, and the input charge affected by input noises of (b) and (c) of FIG. 5 are respectively demonstrated at 600 to 604. The input noise follows IEC 61000-4-6 level 3 standards.

Both the mathematical derivation and simulation show that the impact of the input noise can be reduced and/or eliminated by performing two conversions with sample clock shifted by half cycle.

In a preferred embodiment, the determining unit 118 is also coupled to a second sample capacitor 110 b and determines the touch or proximity to the second capacitive sensor 102 b in a same way as determining the touch or proximity to the first capacitive sensor 102 a.

Referring back to FIG. 1 , different from sequential scanning configuration, in which the touch or proximity is determined based on capacitance change of a single capacitive sensor that correspond to a single button, the sensing system 100 can also be operated in parallel scanning configuration, the touch or proximity is determined based on capacitance changes of both the first and second capacitive sensors 102 a and 102 b that correspond to a single button.

The determining unit 118 generates a first signal N_(A) indicative of a number of the cycles for charging a voltage difference across the first sample capacitor 110 a to the first trip voltage V_(trip1), and a second signal N_(B) indicative of a number of the cycles for charging a voltage difference across the second sample capacitor 110 b to a second trip voltage V_(trip2). In a preferred embodiment, the first and second trip voltages V_(trip1) and V_(trip2) are equal to a same trip voltage V_(trip). The touch or proximity to both the first and second capacitive sensors 102 a and 102 b is determined based on a final count N, which is a combination of the first and second signals N_(A) and N_(B). In one example, the final count N is defined in accordance with the equation below:

$\begin{matrix} {{N\text{=2}N_{A}N_{B}}/\left( {N_{A} + N_{B}} \right)} & \text{­­­(7)} \end{matrix}$

Referring back to FIG. 2 , because the charging and transferring cycles of the second capacitive sensor 102 b is half-cycle phase shifted with respect to the cycles of the first capacitive sensor 102 a. As discussed above, the impact of the input noise is reduced and/or eliminated.

FIG. 7 is a timing diagram 700 demonstrating changes of the voltages V_(X_A) and V_(X_B) at the first and second input terminals 104 a and 104 b during operation of the sensing system 100 of FIG. 1 in accordance with another example.

The timing diagram 700 demonstrates the voltages V_(X_A) and V_(X_B) respectively at 702 and 704, the first and second charging signals S_(1_A) and S_(1_B) respectively at 706 and 708, and the first to fourth transferring signals S_(2_A), S_(3_A), S_(2_B) and S_(3_B) respectively at 710 to 716. Different from the timing diagram 200 of FIG. 200 , the first and second charging signals S_(1_A) and S_(2_B) are synced, the first and third transferring signals S_(2_A) and S_(2_B) are synced and the second and fourth transferring signals S_(3_A) and S_(3_B) are synced and alternated with the first and third transferring signals S_(2_A) and S_(2_B). In a preferred embodiment, a gap time 718 is provided between charging and transferring to ensure break-before-make switch action.

FIG. 8 is a schematic block diagram of a sensing system 800 in accordance with a second embodiment of the present disclosure.

The system 800 is substantially similar to the system 100 of FIG. 1 , except that the first and second transferring units 806 a and 806 b are respectively directly coupled to the first and second sample capacitors 910 a and 910 b, and the first switching unit 808 a is configured to alternately electrically couple one of the first and second transferring units 806 a and 806 b to the first input terminal 804 a. In a preferred embodiment, each of the first and second transferring units 806 a and 806 b is a current mirror. In a preferred embodiment, the current mirror is a class AB current mirror. In a preferred embodiment, the determining unit 818 generates a first signal N₁ indicative of a number of the cycles for charging a voltage difference across the first sample capacitor 810 a to a first trip voltage V_(trip1) in the first conversion, a second signal N₂ indicative of a number of the cycles for charging a voltage difference across the second sample capacitor to a second trip voltage V_(trip2) In a preferred embodiment, the first and second trip voltages V_(trip1) and V_(trip2) are equal to a same trip voltage V_(trip). The touch or proximity to the capacitive sensor 802 a is determined based on a final count N, which is a combination of the first and second signals N₁ and N₂. In one example, the final count N is defined in accordance with the equation below:

$\begin{matrix} {{N = \left( {N_{1} + N_{2}} \right)}/2} & \text{­­­(8)} \end{matrix}$

In addition, as discussed above, the sensing system 800 can be configured to perform two conversions with half-cycle phase shifted to eliminate the impact of the input noise.

Referring to FIG. 9 , a schematic block diagram of a sensing system 900 for sensing touch or proximity to first and second capacitive sensors 902 a and 902 b in accordance with a third embodiment of the present disclosure is shown. The sensing system 900 is substantially similar to the sensing system 100 of FIG. 1 , except that the configuration supports 2 IOs with only 1 transferring unit. The sensing system 900 includes first and second input terminals 904 a and 904 b for receiving charges respectively from the first and second capacitive sensors 902 a and 902 b, and a transferring unit 906 a for transferring the charges received at the first and second input terminals 904 a and 904 b. In a preferred embodiment, the transferring units 906 a is a current mirror. In a preferred embodiment, the current mirror is a class AB current mirror. The sensing system 900 also includes first and second switching units 908 a and 908 b configured to electrically couple the transferring unit 906 a alternately between the first input terminal 902 a and a first sample capacitor 910 a, and between the second input terminal 904 b and a second sample capacitor 910 b.

In a preferred embodiment, the sensing system 900 includes first and second charging switches 912 a and 912 b respectively coupled between a voltage source V_(reg) 914 and the first and second input terminals 904 a and 904 b. The first switching unit 908 a includes first and second transferring switches 916 a and 916 b respectively coupled between the first and second input terminals 904 a and 904 b and the transferring unit 906 a, and the second switching unit 908 b includes third and fourth transferring switches 916 c and 916 d respectively coupled between the transferring unit 906 a and the first and second sample capacitors 910 a and 910 b. The first and second charging switches 912 a and 912 b can be transistors operating between ON and OFF status respectively controlled by corresponding first and second charging signals S_(1_A) and S_(1_B), the first and third transferring switches 916 a and 916 c can be transistors operating between ON and OFF status controlled by a first transferring signal S_(2_A), and the second and fourth transferring switches 916 b and 916 d can be transistors operating between ON and OFF status controlled by a second transferring signal S_(2_B).

FIG. 10 is a timing diagram 1000 demonstrating changes of voltages V_(X_A) and V_(X_B) at the first and second input terminals 904 a and 904 b during operation of the sensing system 900 of FIG. 9 in accordance with an example.

The timing diagram 1200 demonstrates the voltages V_(X_A) and V_(X_B) at the first and second input terminals 904 a and 904 b of the sensing system 900 respectively at 1002 and 1004, the first and second charging signals S_(1_A) and S_(1_B) respectively at 1006 and 1008, and the first and second transferring signals S_(2_A) and S_(2_B) respectively at 1010 and 1012. In an example, the first and second charging signals S_(1_A) and S_(1_B) configure the first and second capacitive sensors 902 a and 902 b to be charged respectively in first and second half of each cycle. The first transferring signal S_(2_A) configures the transferring unit 906 a to transfer the charges from the first input terminal 904 a to the first sample capacitor 910 a in the second half of each cycle, and the second transferring signal S_(2_B) configures the transferring unit 906 a to transfer the charges from the second input terminal 904 b to the second sample capacitor 910 b in the first half of each cycle. In a preferred embodiment, a gap time 1014 is provided between charging and transferring to ensure break-before-make switch action.

Similarly, as discussed above, touch or proximity to each of the first and second capacitive sensors 902 a and 902 b can be determined based on two conversions with half-cycle phase shifted to eliminate the impact of the input noise, and similarly, as discussed above, with parallel scanning configuration, the sensing system 900 can be configured to detect one touch or proximity based on capacitance changes of both the first and second capacitive sensors 902 a and 902 b.

FIG. 11 is a schematic block diagram of a sensing system 1100 in accordance with a fourth embodiment of the present disclosure. The sensing system 1100 is substantially similar to the system 100 of FIG. 1 , except that the system 1100 senses the changes of the capacitance of the first and second capacitive sensors 1102 a and 1102 b based on a mutual capacitance sensing mode, while the sensing system 100 of FIG. 1 senses the changes of the capacitance of the first and second capacitive sensors 102 a and 102 b based on a self-capacitance sensing mode. In the self-capacitance sensing mode, a first end of a capacitive sensor is coupled to a voltage source V_(reg) in charging phases and to a transferring unit in transferring phases, and an opposite second end of the capacitive sensor is coupled to ground. In the mutual capacitance sensing mode, a first end of a capacitive sensor is coupled to a transferring unit, and an opposite second end is coupled to ground in charging phases and to a voltage source V_(reg) in transferring phases.

As shown in FIG. 11 , a first end of the first capacitive sensor 1102 a is coupled to the first input terminal 1104 a of the system 1100. The system 1100 further includes a first charging switch 1112 a coupled between an opposite second end of the first capacitive sensor 1102 a and ground V_(GND), and a first pumping switch 1122 a coupled between the second end of the first capacitive sensor 1302 a and a voltage source V_(reg) 1114, wherein the first charging switch 1112 a and the first pumping switch 1122 a are reciprocally switched on and off to hold a voltage V_(X_A) of the first input terminal 1104 a at a first predetermined voltage V_(tn) in charging phases and pumping the voltage V_(X_A) at the first input terminal 1104 a to a second predetermined voltage in transferring phases. The second predetermined voltage is proportional to a combination of the first predetermined voltage and the voltage source V_(reg).

The system 1100 further includes a first holding and sampling unit 1124 a coupled between the first input terminal 1104 a and an input terminal of the first transferring unit 1106 a, and a second holding and sampling unit 1124 b coupled between the second input terminal 1104 b and an input terminal of the second transferring unit 1106 b. Each of the first and second holding and sampling units 1124 a and 1124 b is configured to sample a bias voltage at the input terminal of the corresponding first or second transferring unit 1306 a or 1306 b in the transferring phases, wherein the first predetermined voltage V_(tn) is based on the bias voltage, and hold the first predetermined voltage V_(tn) in the charging phases. In a preferred embodiment, each of the first and second transferring units 1106 a and 1106 b is a current mirror. In a preferred embodiment, the current mirror is a class AB current mirror. The bias voltage V_(tn) is generated by a bias current proportional to a reference voltage V_(ref).

The system 1100 also includes a first switching unit 1108 a coupled between the first input terminal 1104 a and the first and second holding and sampling units 1124 a and 1124 b, and a second switching unit 1108 b. The first switching unit 1108 a is configured to selectively electrically couple the first input terminal 1104 a to one of the first and second holding and sampling units 1124 a and 1124 b in the charging phases to hold the voltage V_(X_A) at the first predetermined voltage V_(tn). In the transferring phases, similar to the sensing system 100 of FIG. 1 , the first and second switching units 1108 a and 1108 b are configured to alternately electrically couple one of the first and second transferring units 1106 a and 1106 b between the first input terminal 1104 a and a first sample capacitor 1110 a to determine the touch or proximity to the first capacitive sensor 1102 a. The touch or proximity to the first capacitive sensor 1102 a is determined by the determining unit 1118 in a same manner as the touch or proximity to the first capacitive sensor 102 a determined by the determining unit 118 of the system 100 of FIG. 1 , or the touch or proximity to the capacitive sensor 802 a determined by the determining unit 818 of the system 800 of FIG. 8 .

In a preferred embodiment, each of the first and second holding and sampling units 1124 a and 1124 b includes a holding switch 1326 a/1326 b and a sampling switch 1328 a/1328 b. The sampling and holding switches can be transistors operating between ON and OFF status respectively controlled by corresponding holding and sampling signals S_(H_1), S_(S_1,) S_(H_2), S_(S_2) generated by the sensing system 1100. The first charging switch 1112 a and the first pumping switch 1122 a can also be transistors operating between ON and OFF status respectively controlled by corresponding first charging signal S_(CH_A) and transferring signal S_(TR_A) generated by the sensing system 1100.

In a preferred embodiment, the touch or proximity to a second capacitive sensor 1102 b coupled to a second input terminal 1104 b of the system 1100 is determined in a same manner by alternately configuring one of the first and second holding and sampling units 1124 a and 1124 b to hold the voltage V_(X_B) at the second input terminal 11304 b at the first predetermined voltage V_(tn) in the charging phases and reciprocally electrically coupling one of the first and second transferring units 1306 a and 1306 b between the second input terminal 1104 b and a second sample capacitor 1110 b. In a preferred embodiment, the charging phases of the second capacitive sensor 1302 b are synchronized with the charging phases of the first capacitive sensor 1302 a. In another preferred embodiment, the charging phases of the second capacitive sensor 1302 b are half-cycle phase shifted with reference to the charging phases of the first capacitive sensor 1302 a.

Similar to the first and second switching units 118 a and 118 b of the system 100 of FIG. 1 , the first switching unit 1118 a includes first to fourth coupling switches 1116 a to 1116 d coupled between the first number of input terminals 1104 and the second number of transferring units 1106, configured to selectively electrically couple any one of the first and second input terminals 1104 a and 1104 b to any one of the first and second transferring units 1106 a and 1106 b. The second switching unit 1118 b includes and fifth to eighth coupling switches 1116 e to 1116 h coupled between the second number of transferring units 1106 and first and second sample capacitors 1110 a and 1110 b. The first to fourth coupling switches 1116 a to 1116 d can be transistors operating between ON and OFF status respectively controlled by corresponding first to fourth coupling signals S_(1_A), S_(2_A), S_(1_B) and S_(2_B) generated by the sensing system 1100, and the fifth to eighth coupling switches 1116 e to 1116 h can be transistors operating between ON and OFF status respectively controlled by corresponding first to fourth coupling signals S_(3_A), S_(4_A), S_(3_B) and S_(4_B) generated by the sensing system 1100. The determining unit 1118 determines touch or proximity to the first and/or second capacitive sensors 1102 a and 1102 b in a same manner as the determining unit 118 of the sensing system 100 of FIG. 1 . Alternately configuring one of the first and second transferring units 1106 a and 1106 b to transfer the charges from each of the first and second capacitive sensor 1102 a and 1102 b averages distortion caused by each of the first and second transferring units 1106 a and 1106 b, thereby improving the accuracy of capacitance change sensing. In addition, as discussed above, the sensing system 1100 can be configured to perform two conversions with half-cycle phase shifted to eliminate the impact of the input noise.

FIG. 12 is a timing diagram 1200 illustrating changes of voltages V_(X_A) and V_(X_B) at the first and second input terminals 1102 a and 1102 b during operation of the sensing system 1100 of FIG. 11 in accordance with an example. The timing diagram 1200 demonstrates the voltages V_(X_A) and V_(X_B) respectively at 1202 and 1204, the first and second charging signals S_(CH_A) and S_(CH_B) respectively at 1206 and 1208, the first and second transferring signals S_(TR_A) and S_(TR_B) respectively at 1210 and 1212, the first to fourth coupling signals S_(1_A), S_(2_A), S_(1_B) and S_(2_B) respectively at 1214 to 1220, the first holding signal and sampling signal S_(H_1) and S_(S_1) respectively at 1222 and 1224, and the second holding signal and sampling signal S_(H_2) and S_(S_2) respectively at 1426 and 1428, and the fifth to eighth coupling signals S_(3_A), S_(4_A), S_(3_B) and S_(4_B) respectively at 1230 to 1236. The charging phases of the second capacitive sensor 1202 b are synchronized with the charging phases of the first capacitive sensor 1102 a. A gap time 1438 is provided between charging and transferring to ensure break-before-make switch action.

FIG. 13 is a timing diagram 1300 illustrating changes of voltages at the first and second input terminals 1102 a and 1102 b of the sensing system 1100 of FIG. 13 under the control of a controller 1108 in accordance with another example. The timing diagram 1300 is substantially similar to the timing diagram 1200 except that the charging phase of the second capacitive sensor 1102 b is half-cycle phase shifted with reference to the charging phase of the first capacitive sensor 1102 a.

Referring to FIG. 14 , a flow chart of a method 1400 for sensing touch or proximity to a first capacitive sensor with a sensing system in accordance with a fifth embodiment of the present disclosure is shown. The sensing system, with reference to the sensing system 100 of FIG. 1 , includes first and second switching units 108 a and 108 b configured to selectively electrically couple any one of the second number of transferring units 106 to any one of the first number of input terminals 104 of the sensing system 100, wherein at least one of the first and second numbers is equal to or greater than two. In the example shown in FIG. 1 , the sensing system 100 includes first and second input terminals 104 a and 104 b, and first and second transferring units 106 a and 106 b.

Starting at step 1402, where a first conversion is performed. The sensing system 100 configures the first capacitive sensor 102 a of FIG. 1 to be repeatedly charged and discharged in successive cycles, wherein when discharging the first capacitive sensor 102 a includes alternately configuring one of the first and second transferring units 106 a and 106 b to transfer charges from the first capacitive sensor 102 a through the first input terminal 104 a to a first sample capacitor 110 a.

The charging and transferring cycle repeats until a voltage difference across the first sample capacitor 110 a reaches a trip voltage V_(trip), determined at step 1404 by a determining unit 118.

At step 1406, the determining unit 118 generates a first signal N_(1x) indicative of a number of the cycles in the first conversion.

At step 1408, a second conversion is performed. The second conversion is similar to the first conversion except that the charging and transferring phases of the second conversion are half-cycle phase shifted with reference to those of the first conversion.

The charging and transferring cycle repeats until a voltage difference across the first sample capacitor 110 a again reaches the trip voltage V_(trip), determined at step 1410 by the determining unit 118.

At step 1412, the determining unit 118 generates a second signal N_(2x) indicative of a number of the cycles in the first conversion.

At step 1414, the determining unit 118 determines the touch or proximity to the first capacitive sensor 102 a based on a final count N_(x) which is a combination of the first and second signals N_(1x) and N_(2x). In one example, the final count N_(x) is defined in accordance with the equation (1).

In a preferred embodiment, the touch or proximity to a second capacitive sensor 102 b coupled to the second input terminal 104 b of the sensing system 100 is determined based on a combination of third and fourth signals generated in a same manner, wherein the first and second switching units 108 a and 108 b reciprocally configure each of the first and second transferring unit 106 a and 106 b to alternately transfer charges from the first capacitive sensor 102 a to the first sample capacitor 110 a, and from the second capacitive sensor 102 b to the second sample capacitor 110 b.

In another preferred embodiment, the determining unit 118 determines the touch or proximity to both the first and second capacitive sensors 102 a and 102 b based on a combination of the first and third signals.

Referring to FIG. 15 , a flow chart of a method 1500 for sensing touch or proximity to a capacitive sensor with a sensing system in accordance with a sixth embodiment of the present disclosure is shown. The sensing system, with reference to the sensing system 800 of FIG. 8 , includes a first switching unit 808 a configured to selectively electrically couple any one of the second number of transferring units 806 to transfer charges from any one of the first number of input terminals 804 of the sensing system 800, wherein at least one of the first and second numbers is equal to or greater than two. In the example shown in FIG. 9 , the sensing system 800 includes one input terminal 804 a, and first and second transferring units 806 a and 806 b.

Starting at step 1502, where a first conversion is performed. The sensing system 800 configures the capacitive sensor 802 a to be repeatedly charged and discharged in successive cycles, wherein discharging the first capacitive sensor 802 a includes alternately configuring the first transferring unit 806 a to transfer charges 804 a from the capacitive sensor 802 a to the first sample capacitor 810 a, and the second transferring unit 806 b to transfer charges from the first capacitive sensor 802 a to the second sample capacitor 810 b.

The charging and transferring cycle repeats until a voltage difference across the first sample capacitor 810 a reaches a first trip voltage V_(trip1) and a voltage difference across the second sample capacitor 810 b reaches a second trip voltage V_(trip2) determined at step 1504 by a determining unit 818. In a preferred embodiment the first and second trip voltages V_(trip1) and V_(trip2) are equal to a same predetermined trip voltage V_(trip).

At step 1506, the determining unit 818 generates a first signal N₁ indicative of a number of the cycles to charge the voltage difference across the first sample capacitor 810 a to the first trip voltage V_(trip1), and a second signal N₂ indicative of a number of the cycles to charge the voltage difference across the second sample capacitor 810 b to the second trip voltage V_(trip2).

At step 1508, the determining unit 818 determines the touch or proximity to the first capacitive sensor 802 a based on a final count N which is a combination of the first and second signals N₁ and N₂. In one example, the final count N is defined in accordance with the equation (8).

Referring to FIG. 16 , a flow chart of a method 1600 for sensing touch or proximity to first and second capacitive sensors with a sensing system in accordance with a seventh embodiment of the present disclosure is shown. The sensing system, with reference to the sensing system 900 of FIG. 9 , includes first and second input terminals 904 a and 904 b respectively coupled to first and second capacitive sensors 902 a and 902 b, and one transferring unit1 906 a.

Starting at step 1602, the sensing system 900 configures the first and second capacitive sensors 902 a and 902 b to be alternately charged and discharged in successive cycles, wherein discharging the first and second capacitive sensors 902 a and 902 b includes configuring the transferring unit 906 a to alternately transfer charges from the first capacitive sensor 902 a through the first input terminal 904 a to the first sample capacitor 910 a and from the second capacitive sensor 902 b through the second input terminal 904 b to the second sample capacitor 910 a.

The charging and transferring phases of the first and second capacitive sensors 902 a and 902 b repeat respectively until a voltage difference across the corresponding sample capacitor 910 a/910 b reaches a trip voltage V_(trip), determined at step 1604 by a determining unit 918.

At step 1606, the determining unit 918 generates first and second signals N_(A) and N_(B), wherein the first signal N_(A) is indicative of a number of the cycles to charge a voltage difference across the first sample capacitor 910 a to the trip voltage V_(trip), and the second signal N_(B) is indicative of a number of the cycles to charge a voltage difference across the second sample capacitor 910 b to the trip voltage V_(trip) in the first conversion.

At step 1808, the determining unit 918 determines the touch or proximity to the first and second capacitive sensors 1102 a and 1102 b respectively based on the first and second signals N_(A) and N_(B). In another example, in parallel scanning configuration, the determining unit 918 determines the touch or proximity to both of the first and second capacitive sensors 1102 a and 1102 b based on a combination of the first and second signals N_(A) and N_(B) in accordance with the equation (7).

Similarly, as discussed above, touch or proximity to each of the first and second capacitive sensors 902 a and 902 b can be determined based on two conversions with half-cycle phase shifted to eliminate the impact of the input noise.

The description of the preferred embodiments of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or to limit the invention to the forms disclosed. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the present disclosure, as described in the claims. 

1. A method comprising: generating a first signal indicative of a number of cycles for charging a voltage difference across a first capacitor to a first voltage; generating a second signal indicative of a number of cycles for charging a voltage difference across the first capacitor to a second voltage; and determining touch or proximity to a first capacitive sensor based on a combination of the first and second signals.
 2. The method of claim 1, wherein determining the touch or proximity to the first capacitive sensor further comprises: determining touch or proximity to a second capacitive sensor based on a combination of the first and second signals.
 3. The method of claim 2, further comprising: charging a voltage difference across the first capacitor to the first voltage in a first conversion, wherein the number of cycles indicated in the first signal corresponds to the first conversion; and charging the voltage difference across a second capacitor to the second voltage in a second conversion, wherein the number of cycles indicated in the second signal corresponds to the second conversion.
 4. The method of claim 3, wherein cycles of the second capacitor are half-cycle phase shifted from cycles of the first capacitor.
 5. The method of claim 1, wherein the first voltage equals the second voltage.
 6. The method of claim 1, wherein the combination of the first and second signals is based on an equation: N = 2NANB/(NA + NB). .
 7. The method of claim 1, wherein the combination of the first and second signals is based on an equation: N = (N1 + N2)/2. .
 8. A method, comprising: repeatedly charging and discharging a first capacitor in successive cycles during a first conversion until a voltage difference across a second capacitor reaches a voltage threshold, wherein the first capacitor is coupled to the second capacitor; generating a first signal indicative of a number of cycles in the first conversion; repeatedly charging and discharging the first capacitor in successive cycles during a second conversion until a voltage difference across the second capacitor reaches the voltage threshold, wherein charging and discharging the first capacitor during the second conversion is phase shifted a half-cycle from the charging and discharging the first capacitor during the first conversion; generating a second signal indicative of a number of cycles in the second conversion; and determining touch or proximity to the first capacitor based on the first signal and the second signal.
 9. The method of claim 8, wherein the first capacitor is external to a circuit comprising the second capacitor.
 10. The method of claim 9, wherein a transferring unit is coupled between the first capacitor and the second capacitor.
 11. The method of claim 9, wherein a first switching circuit, a second switching circuit, and a transferring unit is coupled between the first capacitor and the second capacitor.
 12. The method of claim 8, further comprising: determining touch or proximity to a third capacitor based on a third signal and a fourth signal, wherein: the third capacitor is coupled to the second capacitor through at least one of a first switching circuit or a second switching circuit and at least one of a first transferring unit or a second switching unit; and the third capacitor is coupled to a fourth capacitor through the first switching circuit, the second switching circuit, and the second transferring unit.
 13. A method comprising: during a first conversion: repeatedly charging and discharging a first capacitor in successive cycles; determining whether a first voltage difference across a second capacitor reaches a first voltage; determining whether a second voltage difference across a third capacitor reaches a second voltage; generating a first signal indicating a number of the successive cycles to charge the first voltage difference to the first voltage generating a second signal indicating a number of successive cycles to charge the second voltage difference to the second voltage; and determining touch or proximity to the first capacitor based on the first signal and the second signal.
 14. The method of claim 13, wherein discharging the first capacitor comprises alternately configuring: a first transferring unit to transfer charge from the first capacitor to the second capacitor; and a second transferring unit to transfer charge from the first capacitor to the third capacitor.
 15. The method of claim 13, wherein the first voltage and the second voltage are predetermined.
 16. The method of claim 13, wherein: the first capacitor is coupled to a switching circuit having a first output and a second output; the first output of the switching circuit is coupled to a first transferring unit; the second output of the switching circuit is coupled to a second transferring unit; the first transferring unit is coupled to the second capacitor; and the second transferring unit is coupled to the third capacitor.
 17. The method of claim 16, wherein the switching circuit comprises a first switch coupled to the first transferring unit, and a second switch coupled to the second transferring unit.
 18. The method of claim 16, wherein the first transferring unit and the second transferring unit are each coupled to a reference voltage.
 19. The method of claim 16, further comprising a switch configured to couple a voltage source to the first capacitor and to the switching circuit.
 20. The method of claim 13, wherein: determining whether the first voltage difference across the second capacitor reaches the first voltage comprises comparing the first voltage difference to the first voltage using a first comparator; and determining whether the second voltage difference across the third capacitor reaches the second voltage comprises comparing the second voltage difference to the second voltage using a second comparator. 